[FIX] AccessEquipmentRef incorrently subsitutes EquipmentRef

[skinkie]

AccessEquipment subsitutes InstalledEquipment, allowing a StaircaseEquipment or LiftEquipment to be added to a SitePathLink. Incorrectly, an StaircaseEquipmentRef may not be used due to EquipmentRef is subsituted instead of InstalledEquipmentRef.

[skinkie]

@ue71603 can you validated if this is fixed by #293? If so, #168 may be closed. Notice that @Aurige then did a different approach for resolving it. I took the transmodel approach by making it an installed equipment. We may need to discuss this.